The tumor suppressor p53 plays a central role in a process by which a cell senses and responds to a variety of stresses. It acts as a critical monitor preventing the survival of cells with genetic, and possibly, other types of permanent damage which are beyond repair. These responses prevent the accumulation of damaged and transformed cells and protect the organisms from processes that could lead to malignant formation. The key properties of p53 clearly indicate its importance in the cellular stress and damage responses of the cells. The levels of p53 are tightly regulated in normal unstressed cells by the MDM2 gene product, which binds p53 and targets p53 for ubiquitin-mediated degradation by the proteasome. DNA-damaging agents (e.g. ionising or non-ionizing radiation and cytotoxic drugs) and other types of stresses (e.g. hypoxia and oncogenes) activate p53 leading to activation of the p53 transcriptional program. This includes transactivation of a vast number of the p53 downstream target genes such as p21WAF1/CIP1 (p21) and a number of apoptosis associated targets (e.g. PUMA, NOXA, PIGs etc.), many of which are yet to be characterized (Zhao et al, 2000; Wang et al, 2001; Wei et al., 2006). p21 negatively regulates kinases involved in cell cycle progression by abrogating G1/S cell cycle transition to provide time for DNA damage repair before cell division. In the presence of irrepairable damage, p53 may induce apoptosis to eliminate damaged cells. In the absence of p53 function, DNA alterations accumulate, leading to initiation or progression of malignancy.
The p53 family includes thus far the founding member, p53, and the more recently recognized closely related members, p63, p73, as well as, numerous isoforms and alternatively spliced versions of each isoform (Murray-Zmijewski et al., 2006). p63 and p73 share many structural and functional features with p53, and activate an overlapping set of genes with p53. p63 is essential in epidermal morphogenesis and limb development, while its relevance in cancer is unclear (Melino et al., 2003). p73 is known to be essential in neuronal development, inflammatory responses and reproductive behaviour (Murray-Zmijewski et al., 2006; Levrero et al., 2000). Both p63 and p73 are capable of activating a cell cycle arrest and apoptotic program akin to that induced by p53.
A prerequisite for p53-mediated transcriptional activation is its ability to specifically bind to a responsive element (RE) within downstream gene promoters that comprises two copies of the palindromic consensus sequence “PuPuPuC(A/T)(T/A)GpyPyPy”, separated by 0-13 by (el-Deiry et al., 1992). This same responsive element is also utilized by both p63 and p73 (Murray-Zmijewski et al., 2006; Levrero et al., 2000). After binding to the RE, p53 stimulates the assembly of the transcription preinitiation complex TFIID-TFIIA at the TATA element (Xing et al., 2001). Most of p53 mutations found in human tumors are located in the p53 central domain and affect amino acid residues which contact DNA or are required for the correct folding of p53, thus abolishing the RE binding and transcriptional activation (Cho et al., 1994). p53 has been shown to bind its RE exclusively as a tetramer (Friedman et al., 1993). However, most of the natural p53 REs do not fit completely the consensus sequence (Kim et al., 1997), and other factors, such as p53 modifications, chromatin modifications and folding, and binding of other co-regulatory factors at specific promoter sites, determine the extent of transcriptional response. This also mechanism of action also applies to both p63 and p73 and both are capable of utilizing the RE-element for sequence specific binding, they also regulate a large set of targets specific for each isoform.
Over 50% of human tumors harbor TP53 mutations, which render the p53 protein functionally impaired. This makes TP53 the most commonly mutated gene in human cancers. It is also speculated that the remaining human tumors have dysfunctional p53, through disturbed regulation of the p53 pathway or protein or modifiers of the p53 protein-protein interactions. The essential role of p53 in tumor suppression is evidenced also by TP53 germline mutations, which cause a rare hereditary cancer predisposition, Li-Fraumeni syndrome, and in p53-deficient animal models having severely enhanced tumorigenesis.
Due to its apoptosis inducing properties, p53 is an extremely attractive target for drug development. One particular drug compound, nutlin-3a (Vassilev et al., 2004), is a highly specific activator of p53, and we have established that nutlin-3a is the first and highly effective agent inducing B-cell lymphoma (Kaposi's sarcoma herpes virus-infected pleural effusion lymphoma) cell killing both in vitro and in vivo mouse models (Sarek et al., 2007). Furthermore, activation of the p53 pathway using nutlin-3a does not cause any adverse side-effects in the mouse. Based on these studies it has been concluded that inactivation of the p53 pathway by the KSHV virus lies in the pathogenesis of this incurable malignancy.
Considering that                i) p53 is a highly responsive molecule to cellular stress and DNA damage, implicated in diverse diseases like cancer, ischemia, neuronal disorders, inflammation and also during physiological processes like in normal cellular metabolism, development and aging (Vousden and Lane, 2007),        ii) p53 is a key activator of cell cycle arrest/apoptosis pathways and pathways implicated in the abovementioned processes,        iii) its mutations are uncommon in several common chemotherapy-resistant human cancers (e.g. melanoma, prostate cancer),        iv) its activation in conditional mouse models has significant potency in causing tumor regression (Martins et al. 2006, Ventura et al. 2007, Xue et al. 2007), and        v) that pilot studies using p53-activating compounds such as nutlin-3a (Vassilev et al. 2004, Sarek et al., 2007) show remarkable in vivo anti-tumor effects without side-effects, further development of small-molecule compounds affecting p53 pathway is highly warranted and has potential for therapeutic exploitation.        